Computer disk drives store information on magnetic disks in concentric tracks. A typical computer disk drive is illustrated in FIG. 1. The disk drive, generally identified by reference numeral 100, includes a base 104 and magnetic disks 108 (only one of which is shown in FIG. 1). The magnetic disks 108 are interconnected to the base 104 by a spindle motor (not shown) mounted within or beneath the hub 112, such that the disks 108 can be rotated relative to the base 104. Actuator arm assemblies 116 (only one of which is shown in FIG. 1) are interconnected to the base 104 by a bearing 120, such that the actuator arm assemblies 116 can be moved radially with respect to the magnetic disks 108. The actuator arm assemblies 116 each include a transducer head 124 at a first end, to address each of the surfaces of the magnetic disks 108. A voice coil motor 128 pivots the actuator arm assemblies 116 about the bearing 120, to radially position the transducer heads 124 across the surfaces of the magnetic disks 108. The voice coil motor 128 is operated by a controller 132 that is in turn operatively connected to a host computer (not shown). By changing the radial position of the transducer heads 124 with respect to the magnetic disks 108, the transducer heads 124 can access different tracks or cylinders 136 on the magnetic disks 108.
The high rotational speed of the magnetic disks 108 when the disk drive 100 is in use creates a boundary layer of air that rotates with the surface of each disk 108. This boundary layer is sufficient to suspend the transducer heads 124 above the surfaces of the disks 108 at a predetermined flying height. As the storage capacities of hard disk drives have increased, the flying height of the transducer heads 124 has become increasingly small. A low flying height assists in increasing the storage density of a drive 100 by allowing the magnetic transitions that store information on the disks 108 to be more tightly grouped. However, a low flying height requires a smooth disk surface, which results in increased friction between the transducer heads 124 and the surfaces of the disks 108 when the disks 108 are not rotating, thereby making it more difficult to bring the disks 108 to a rotational speed at which the heads 124 can fly. In certain instances, a “stiction event” can occur, in which the torque of the spindle motor is insufficient to break the adhesion between the transducer heads 124 and the surfaces of the disks 108. In order to overcome these problems, disk drives have been provided with special “landing zones” having a textured surface and designed for receiving the transducer heads 124 when the disks 108 are not rotating. However, these textured areas can cause oscillations in low flying heads 124. In addition, the provision of landing zones does not prevent actuator arms 116 from moving and coming into contact with the disks 108, for instance in response to shocks, and damaging the surfaces of the disks 108.
In order to overcome these problems, disk drive actuator arm assemblies 116 may be provided with tabs or cam followers 138 capable of engaging corresponding cams 140 when the actuator arm assemblies 116 are in a parked position. The cams 140 each generally contain a ramp portion 144 and a detent portion 148. When the disk drive 100 is not in use, the actuator arm assemblies 116 are generally positioned such that the tabs 138 are held in the cams 140 at the detents 148. The transducer heads 124 are said to be “unloaded” from the disks 108 when the tabs 138 are held by the cams 140. The terms “load” and “unload” can be interchanged, but for purposes of the present invention, “unloading” refers to removing a transducer head 124 from the disk 108 surface and “loading” refers to placing a transducer head 124 adjacent the disk 108 surface such that read and write operations may be carried out. When the transducer heads 124 are in the unloaded position, the magnetic disks 108 are protected from damage that may be caused by a collision between a transducer head 124 and the disk 108, because the actuator arms 116 are held in place by the cams 140.
Before data can be read from or written to the disks 108, the transducer heads 124 must be “loaded” onto the surfaces of the magnetic disks 108. In loading the transducer heads 124, it is important to ensure that the transducer heads 124 are not traveling at too great a velocity. If the transducer heads 124 leave the cam 140 at too great a velocity, the component of their motion that is perpendicular to the surfaces of the disks 108 will likely be too great for the boundary layer of air to support the transducer heads 124 and prevent contact between the transducer heads 124 and the disks 108. Such contact will likely cause a loss of data from the disk drive 100. Conversely, it is important to load the transducer heads 124 as quickly as possible, in order to limit the time period during which the host computer must wait before information can be retrieved from the disk drive 100. Accordingly, it is desirable to closely regulate the velocity of the transducer heads 124 during loading. During unloading, high speed is also desirable. However, the transducer heads should not be unloaded at too great a speed, to avoid damaging the actuator arm assemblies when they contact the cams 140. Also, if the head travels at too great a speed, it can bounce and strike the disk. In addition, cam wear is higher if the head travels at too great a velocity.
While the transducer heads 124 are being loaded, the transducer heads 124 are lifted away from the surfaces of the disks 108 by the cam 140. Accordingly, information encoded on the disks 108 concerning the position of the transducer heads 124 with respect to the surfaces of the disks 108 is not available, and the velocity of a transducer head 124 cannot be determined by reading information from the disks 108. However, the movement of the coils of the voice coil motor 128 with respect to the magnets of the voice coil motor 128 produces a back electromagnetic force (BEMF; or back EMF) in the coil of the voice coil motor 128. Because this back EMF is proportional to the velocity of the actuator arm assemblies 116, it can be sensed and used to determine the velocity of the transducer heads 124.
The back EMF generated by the movement of the voice coil motor 128 can be determined if the resistance and inductance of the voice coil motor 128 are known. In particular, the back EMF generated by the movement of the voice coil motor 128 is equal to the voltage supplied to the voice coil motor less the voltage drop due to the internal resistance and inductance of the voice coil motor. However, this method is unreliable, as the resistance of the voice coil motor 128 changes while the voice coil motor 128 is in motion.
Another approach to reading the back EMF generated in a voice coil motor 128 has been to turn off the drive current to the voice coil motor 128 at regular intervals of time. The back EMF is then sampled while the drive current is off. One such prior art approach is depicted in FIG. 2. In FIG. 2, a train of pulses 200 having regular pulse widths Tp 204 and varying current or voltage levels are shown. In between the pulses 200 are regular sampling intervals TS 208. During the sampling intervals, samples of the back EMF S1, S2, S3, S4 and S5 are taken. According to this approach, the width Tp 204 of the pulses is constant, and the amplitude of the pulses 200 is varied in order to adjust the velocity of the transducer heads 124. The provision of electrical power to the voice coil motor 128 is discontinued during the sampling times TS 208 in order to allow an accurate reading of the back EMF to be taken. However, this approach results in the production of a relatively loud and objectionable audible noise due to the regular frequency with which power is applied to the voice coil motor 128.
Another approach to controlling the velocity with which transducer heads 124 are loaded onto the surface of disks 108 is depicted in FIG. 3. According to this approach, pulses 300 having a first pulse width Tp1 304 and amplitude V1 308 are applied to the voice coil motor 128 in order to maintain a desired velocity of the transducer heads 124 with respect to the surfaces of the disks 108. Where the velocity of the transducer heads 124 is too great, the pulses 300 are discontinued. After a predetermined number of pulses 300 having a first width Tp1 304 have been provided to the voice coil motor 128, pulses 300 having a narrower width Tp2 312 are applied to the voice coil motor 128. As with the wider pulses 300, the pulses having a narrower width Tp2 312 are not provided if the velocity of the transducer heads 124 is found to be too high. Samples of the back EMF are taken at times S1 to S9, when no power is provided to the voice coil motor 128. Accordingly, where the velocity of the transducer heads 124 is at or below a desired velocity, a train of pulses having a regular frequency is produced, thereby creating undesired acoustical noise. In addition, although the provision of pulses may be interrupted where the velocity of the transducer heads 124 is higher than a desired velocity, it is the velocity of the transducer heads 124 which determines whether the pulse train is interrupted or not. Accordingly, this approach does not reliably decrease acoustical noise produced during the loading and unloading of the transducer heads 124.
It would be desirable to provide a method and apparatus for loading and unloading transducer heads 124 from the surfaces of magnetic disks 108 in such a way that an objectionable acoustical output is not produced. In addition, it would be desirable to provide such a method and apparatus that allows accurate control of the velocity with which transducer heads 124 are loaded and unloaded from disk 108 surfaces. Furthermore, it would be desirable to provide such a method and apparatus that is inexpensive to implement and reliable in operation.